Alkali vapor-cells have been used extensively since the 1960s in the study of light-atom interactions. Vapor-cell applications, both proposed and realized, include atomic clocks, communication system switches and buffers, single-photon generators and detectors, gas-phase sensors, nonlinear frequency generators, and precision spectroscopy instrumentation. However, most of these applications, and cold-atom systems in general, have only been created in laboratory settings.
Macroscale vapor cells are widely used in macroscale atomic clocks and as spectroscopy references. Macroscale vapor cells are typically 10-100 cm3 in volume, which is insignificant for m3 scale atomic clocks, but far too large for chip-scale atomic clocks which are at most a few cm3 in volume.
A key driver has thus been to reduce vapor-cell size. Traditional vapor-cell systems are large and, if they have thermal control, have many discrete components and consume a large amount of power. To realize the full potential of vapor-cell technologies, the vapor-cell systems need to be miniaturized.
Chip-scale atomic clocks and navigation systems require miniature vapor cells, typically containing cesium or rubidium, with narrow absorption peaks that are stable over time. Miniature vapor cells, and methods of filling them with alkali metals, have been described in the prior art. However, it has proven difficult to load a precise amount of alkali metal into a miniature vapor cell through the methods described in the literature. Miniature vapor cells have higher surface-area-to-volume ratios than macroscale vapor cells, and are more difficult to load than macroscale vapor cells.
It is difficult to load a precise amount of alkali metal into a miniature vapor cell. Furthermore, the amount of alkali vapor in a vapor cell changes over time as the vapor adsorbs, diffuses, and reacts with the walls. Alkali metal vapor pressure may be changed with a small set of known technologies (see Monroe et al., Phys Rev Lett 1990, 65, 1571; Scherer et al., J Vac Sci & Tech A 2012, 30; and Dugrain, Review of Scientific Instruments, vol. 85, no. 8, p. 083112, August 2014). However, these systems are slow, complex, and/or have a short longevity.
A number of patents and patent applications discuss miniature vapor cells and methods of filling them with alkali metals. See U.S. Pat. No. 8,624,682 for “Vapor cell atomic clock physics package”; U.S. Pat. No. 8,258,884 for “System for charging a vapor cell”; U.S. Pat. No. 5,192,921 for “Miniaturized atomic frequency standard”; WO 1997012298 for “A miniature atomic frequency standard”; and WO 2000043842 for “Atomic frequency standard.”
Traditionally, alkali metals have been introduced into magneto-optical trap (MOT) vacuum systems via difficult-to-control preparation steps, such as manually crushing a sealed alkali-containing glass ampule inside a metal tube connected to the vacuum system via a control valve. See Wieman, American Journal of Physics, vol. 63, no. 4, p. 317, 1995. This approach requires external heating to replenish the alkali metal inside the vacuum system as needed, which is a slow process with little control over the amount of alkali metal delivered. The manual labor is non-ideal for automated systems or chip-scale devices.
An alternative exists in the now-common alkali metal dispensers, which are effectively an oven-controlled source of alkali metal, whereby the desired alkali metal is released by chemical reaction when a current is passed through the device. While this process automates the release of alkali metal into the vacuum system, it has difficulty in fabrication compatibility with chip-scale cold-atom devices. Further, the timescales required for generating (warm up) and sinking (pump down) alkali are typically on the order of seconds to minutes, and can vary greatly depending on the amount of alkali metal built up on the vacuum system walls.
A rapidly pulsed and cooled variant of the alkali metal dispenser has been developed to stabilize the residual Rb vapor pressure in 100 millisecond pump-down time, but the device requires large-dimension Cu heat sinks and complicated thermal design (Dugrain, Review of Scientific Instruments, vol. 85, no. 8, p. 083112, August 2014) which may not easily translate to miniaturization.
Double MOTs wherein the first MOT is loaded at moderate vacuum, and then an atom cloud is transferred to a second high-vacuum MOT, have been implemented on the laboratory scale. Again, these systems require complicated dual-vacuum systems and controls as well as a transfer system to move the atom cloud from one MOT to the other, none of which is amenable to chip-scale integration.
Light-induced atomic desorption (LIAD) is a recent technique that solves some of the long pump-down times by only releasing a small amount of alkali using a desorption laser; however, this method requires preparing a special desorption target in the MOT vacuum chamber. The desorption laser can interfere with the trapping lasers of the MOT (see Anderson et al., Physical Review A, vol. 63, no. 2, January 2001). It also has yet to demonstrate suitable time constants below 1 second.
Thermoelectric stages can be used to regulate the overall temperature of the vapor cell, but this requires the addition of the thermoelectric stages, a temperature sensor and controller, and a significant amount of power (watts) to maintain the entire temperature of the cell at the correct temperature for MOT operation. The effectiveness of this approach will also depend on the overall size of the MOT cell and the efficiency of the thermoelectric stages, limiting the time constants at which the MOT can be loaded and the residual pressure stabilized.
Draper Laboratory has developed a solid-state ionic concept based on Cs conducting glass; see U.S. Pat. No. 8,999,123 and U.S. Patent App. Pub. No. 20110247942. However, the Draper technology has critical features. For example, the Cs conducting glass has low ion conductivity. The implications of this are shown in Bernstein et al., “All solid state ion-conducting cesium source for atomic clocks,” Solid State Ionics Volume 198, Issue 1, 19 Sep. 2011, Pages 47-49, in which >1000 V applied voltage and elevated temperature (˜170° C.) are required to change the alkali content on time scales of ˜100 seconds.
What is instead desired is to work with much lower voltages (such as 1-100 V), lower temperatures (such as 25° C.), and preferably faster time response than 100 seconds. Cold atom lifetime is typically <1 second. The excess atoms should therefore be removed from the vapor chamber on response time scales <1 second in order to have a significant effect on the cold atom lifetime. In some applications, removing atoms from vapor cells on longer time scales is acceptable.
There is also a long-felt need for operation of cold atom systems at elevated temperatures. It has long been desirable to operate cold atoms systems at elevated temperature for precise timing and navigation applications, but the high equilibrium vapor pressure of the alkali metal vapors used at elevated temperatures leads to short lifetimes of the cold atoms (such as less than 1 millisecond), which reduces the stability of the measurement by orders of magnitude.
Draper Laboratory also reports using a thin gold film to enhance alkali transport. Because the diffusion coefficient of alkali atoms in gold is extremely slow, the gold film was made thin with the goal of achieving a porous film. This film only enhanced alkali transport rate by a factor of 2. Other researchers have found that gold can be a source and sink of alkali atoms (Squires, High repetition rate Bose-Einstein condensate production in a compact, transportable vacuum system, Ph.D. Thesis, University of Colorado, 2008). However, the alkali interaction with the gold is a surface effect due to the low diffusivity of alkali atoms in gold.
In Bernstein et al., “SOLID STATE ELECTROCHEMICAL ALKALI SOURCES FOR COLD ATOM SENSING,” Solid-State Sensors, Actuators and Microsystems Workshop, South Carolina, June 2016 (pages 180-184), Draper Laboratory discloses the use of inert Pt electrodes with 130 V to reduce Rb vapor pressure using Rb-beta-alumina. This is a very high required voltage. Furthermore, the backside electrodes contain a metal that is ionized and injected into the solid electrolyte (e.g. Ag) that is different from the desired atoms in the vapor phase of the vapor cell (e.g. Cs or Rb). This contaminates the solid electrolyte and limits the usable lifetime for the device.
Graphite has been used as a sink for stray alkali atoms in atomic systems. Honeywell has employed heated graphite to control the vapor pressure of alkali atom vapor (U.S. Patent App. Pub. No. 20150226669). Using graphite in this manner does not make use of electrochemistry. Alkali atoms merely diffuse into and out of the graphite as the equilibrium vapor pressure of the alkali atom vapor changes, by changing the temperature of the graphite. There are a number of drawbacks to this approach:
1. The alkali vapor pressure is indirectly controlled through temperature.
2. The graphite must be thermally isolated from the rest of the cell, thus increasing device complexity.
3. Heating the graphite will require significant power, thus limiting portability.
4. Heating the device to the appropriate temperature will require temperature monitoring and feedback, which adds further complexity.
An atom chip is an arrangement of microfabricated current-carrying wires patterned on a substrate which is used to trap and control atoms via the strong magnetic field gradients offered at distances close to conductors. Atoms chips enable highly sophisticated experiments to be condensed into areas on the order of a few square centimeters and readily lend themselves to the miniaturization and integration of cold atom systems for practical applications beyond the laboratory.
Atom chips use metal traces patterned via lithographic techniques to create magnetic fields involved in trapping populations of atoms. See U.S. Pat. No. 7,126,112 for “Cold atom system with atom chip wall”; Fortagh et al., Rev. Mod. Phys. 79, 235 (2007) Reichel et al., Atom Chips, Wiley, 2011; and Treutlein, Coherent manipulation of ultracold atoms on atom chips, Dissertation, Ludwig-Maximilians-University Munich, 2008, which is hereby incorporated by reference. Atom chips typically are implemented as one wall of a vapor cell. Thus they suffer from the same issues—such as slow vapor pressure rate of change and loss of alkali vapor to the walls—as conventional vapor cells. Improvements to conventional vapor cells in which magnetic trapping fields are generated by magnets or electromagnets outside the vapor cell also apply to atom chips for which magnetic fields are generated by magnets or electromagnets inside the vapor cell.